The long term goal is to explore the mechanism by which accessory cells process and present antigen to lymphoid cells and how they function in the regulation of the immune response. Recent work under this project has shown that accessory cells which supported antigen specific primed lymph node T cell proliferation were Thy 1.2 negative, radioresistant, glass adherent and expressed both I-A and I-E or C subregion antigens; and that the ability of spleen adherent cells to function as accessory cells in the in vitro primary humoral responses to TNP-(T,G)-A--L and TNP-(H,G)-A--L was controlled by autosomal dominant genes located in the K or I-A regions of the responder H-2 complex and therefore mapped identically to the Ir genes controlling overall in vitro and in vivo responsiveness to these antigens.